Oxybisaniline (OBA) is an important monomer for preparation of polyamide-imides and polyimides. Polytrimellitimides, for example, are prepared by the reaction between an acid halide of a benzene tricarboxylic acid anhydride and an organic diamine such as oxybisaniline. Polytrimellitimides are useful as polymeric wire enamels which exhibit many desirable properties. Polyimides are useful in film and shaped articles for high performance applications.
Oxybisaniline can be prepared in a number of ways, two of the more common methods being the reduction of bis(p-nitrophenyl) ether and the ammonolysis of the corresponding dichloro or dibromo diphenyl ether. In such preparation, because of impurities in the starting material or side reactions which occur during the process, there are produced various undesirable by-products including both monofunctional and difunctional compounds such as phenoxyaniline, halo or nitrophenoxyaniline, aminophenoxyphenol, and the like. Position isomers of oxybisaniline may also be present in the product. The preparation of pure oxybisaniline is also often complicated by the presence in the crude product of highly colored contaminants such as iron compounds or materials formed by decomposition or oxidation of amino groups.
Methods are known whereby such colored impurities can be removed from a product. For example, most or all of the tarry decomposition products can be separated by treating a solution of the impure product with activated carbon. Iron compounds are advantageously reduced to the ferrous state and thereby maintained in solution while the oxybisaniline is precipitated or crystallized and separated by filtration. Such reduction also is effected by reducing some of the colored organic impurities. Compounds such as sodium dithionite, sodium bisulfite, and sodium formaldehyde sulfoxylate are effective reducing agents for this purpose.
It is also known that some by-products, particularly compounds of lower functionality than the desired product, are effectively removed by extracting an aqueous solution of a strong mineral acid salt of oxybisaniline with a water-immiscible solvent such as an aliphatic ketone of five to eight carbon atoms, a polychlorinated lower aliphatic hydrocarbon, or an aromatic hydrocarbon of the benzene series. This extraction procedure is also effective to some extent in removing difunctional impurities from the product. However, complete removal of these materials from oxybisaniline is not possible by this method.
It has now been found that oxybisaniline can be prepared in high yield and a highly purified grade of product can be obtained by reaction of a hydroxylamine and 4,4'-dicarboxydiphenyl ether by means of a Lossen Rearrangement. This reaction is accomplished by reacting the hydroxylamine with 4,4'-dicarboxydiphenyl ether in polyphosphoric acid after a minimum temperature of 140.degree. C. has been reached. At a reaction temperature of less than 140.degree. C., the reaction rate is very slow and conversion of the starting materials to oxybisaniline is not complete. Hydrolysis of the reaction mixture yields oxybisaniline in high yield. Production of by-products as has been the common result in other methods of preparing oxybisaniline is negligible.
The Lossen Rearrangement to convert aromatic acids and their derivatives to amines has been described by Snyder, et al., JACS, 75, 2014 (1953). The reaction, according to Snyder, is carried out by heating a mixture of the reactants, comprising aromatic acids, hydroxylamine and polyphosphoric acid, with stirring, until rapid evolution of carbon dioxide begins, usually in the range of 150.degree. C. to 170.degree. C. At such temperatures, the reaction normally is complete in five to ten minutes, and the mixture is poured over crushed ice to give an aqueous solution of the amine phosphate. The yields of the crude products, as reported by Snyder, ranged from zero to as high as 82%. Snyder explained the range of yields in that acids containing electron-donating substituents were found to give higher yields than those containing electron-withdrawing groups. Melting points of the products indicated the crude products required further purification. Synder reported that acids which gave poor yields of amines produced large amounts of dark-colored acid-insoluble material of very indefinite melting point. Snyder reported that valeric and caprylic acids failed to yield more than mere traces of the expected amines. Caprylohydroxamic acid also failed to yield an amine.